Although scientists observe that an organism’s behavior falls into rhythmic patterns, they disagree about how these patterns are affected when the organism is transported to a new environment. One experimenter, Brown, brought oysters from Connecticut waters to Illinois waters. She noted that the oysters initially opened their shells widest when it was high tide in Connecticut, but that after fourteen days their rhythms had adapted to the tide schedule in Illinois. Although she could not posit an unequivocal causal relationship between behavior and environmental change, Brown concluded that a change in tide schedule is one of several possible exogenous influences (those outside the organism) on the oysters’ rhythms. Another experimenter, Hamner, however, discovered that hamsters from California maintain their original rhythms even at the South Pole. He concluded that endogenous influences (those inside the organism) seem to affect an organism’s rhythmic behavior.
17. All of the following could be considered examples of exogenous influences on an organism EXCEPT the influence of the
(A) level of a hormone on a field mouse’s readiness for mating
(B) temperature of a region on a bear’s hibernation
(C) salt level of a river on a fish’s migration
(D) humidity of an area on a cat’s shedding of its fur（A）
(E) proximity of an owl on a lizard’s searching for food
18. Which of the following statements best describes the conclusion drawn by Brown (lines 14-17)
(A) A change in tide schedule is the primary influence on an oyster’s rhythms.
(B) A change in tide schedule may be an important exogenous influence on an oyster’s rhythms.
(C) Exogenous influences, such as a change in tide schedule, seldom affect an oyster’s rhythms.
(D) Endogenous influences have no effect on an oyster’s rhythms.（B）
(E) Endogenous influences are the only influences on an oyster’s rhythms.
19. The passage suggests that Brown’s study was similar to Hamner’s in which of the following ways?
I. Both experimenters discovered that a new environment had a significant effect on an organism’s behavior rhythms.
II. Both experimenters observed an organism’s behavioral rhythms after the organism had been transported to a new environment.
III. Both experimenters knew an organism’s rhythmic patterns in its original environment.
(A) I only
(B) II only
(C) I and II only
(D) II and III only（D）
(E) I, II, and III
20. Which of the following, if true, would most weaken Brown’s conclusion?
(A) The oyster gradually closed their shells after high tide in Illinois had passed.
(B) The oysters’ behavioral rhythms maintained their adaptation to the tide schedule in Illinois throughout thirty days of observation.
(C) Sixteen days after they were moved to Illinois, the oysters opened their shells widest when it was high tide in Connecticut.
(D) A scientist who brought Maryland oysters to Maine found that the oysters opened their shells widest when it was high tide in Maine.（C）
(E) In an experiment similar to Brown’s, a scientist was able to establish a clear causal relationship between environmental change and behavioral rhythms.
